With developments in optical communication and optical storage devices, demand for semiconductor lasers to be used therefor is growing, and a blue-violet semiconductor laser comprising a III-V group nitride-base semiconductor material such as gallium nitride (AlxGayIn1-x-yN; 0≦x≦1, 0≦y≦1) is actively developed as a key device for realizing a super high density recording on an optical disc. Particularly, increase in output power of the blue-violet semiconductor laser enables high-speed writing in an optical disc, and further, it is a technique indispensable for exploration of a new technical field such as application as a light source to a laser display which can represent bright colors using laser light of a narrow spectrum width.
FIG. 10 is a diagram illustrating an example of a conventional semiconductor laser having a current constriction structure.
The semiconductor laser 100 shown in FIG. 10 includes a semiconductor layer laminated body 110 which is obtained by successively depositing, on an upper surface of an n type GaN substrate 101, an n type AlGaN cladding layer 102, an n type GaN lightguide layer 103, an active layer 104 having a multiquantum well structure including InGaN, an undoped GaN cap layer 105, a p type GaN lightguide layer 106, and a p type AlGaN cladding layer 107.
The p type AlGaN cladding layer 107 is a ridge type cladding layer having a mesa-shaped ridge 107a, whose cross-section perpendicular to the longitudinal direction of the semiconductor laser is a convex trapezoid, and a p type GaN contact layer 108 is disposed on an upper surface of the ridge. Further, an insulating film 111 is deposited on the surfaces of the p type AlGaN cladding layer 107 and the p type GaN contact layer 108, and a p electrode 115 is disposed on the insulating film 111. The p electrode 115 is connected to the p type GaN contact layer 108 through an aperture that is formed in a portion of the insulating film 111 on the p type GaN contact layer 108, along the ridge 107a. An n electrode 116 is disposed on a lower surface of the n type GaN substrate 101.
In the semiconductor laser 100, the ridge 107a, the insulating film 111, the contact layer 108, and the p electrode 115 constitute a stripe structure for injecting current concentratedly into an optical waveguide part of the active layer 104, which is opposite to the ridge 107a. The portion of the active layer 104 opposite to the ridge 107a is an optical waveguide in which laser light is generated and guided, and both end facets perpendicular to the ridge extending direction of the optical waveguide are facets of a resonator.
In the above-described semiconductor laser, when a driving voltage is applied between the p electrode 115 and an n electrode 116, laser driving currents are injected into the active layer 104 from these electrodes. Then, the currents injected into the active layer are concentrated at the optical waveguide part of the active layer 104, which part is opposite to the ridge 107a and constitutes the resonator, and light is generated in the active layer 104. When the injected current exceeds a predetermined threshold value, laser oscillation occurs in the resonator, and laser light is emitted from the resonator facet to the outside.
In order to use the semiconductor laser thus constructed as a light source of a laser display, high output characteristic of 100 mW˜several W is required. On the other hand, a semiconductor laser for a light source of a laser display is not required to have light condensing characteristic up to diffraction limit, which is required of a pickup for an optical disc. Therefore, the semiconductor laser may have a wide-stripe structure having a wide ridge, and a semiconductor laser having high-efficiency and high-output laser characteristic is required. Further, increase in the carrier density in the active layer is required for high output power of the semiconductor laser.
Generally, the stripe width is uniform over the entirety of the resonator. With an increase in the current injected from the electrodes, the carrier density in the active layer increases, and when its value reaches a constant threshold carrier density, laser oscillation occurs. The light output of the laser increases in proportion to the density of carriers which are injected into the active layer, at a current higher than the threshold current. However, when the carrier density inside the active layer is too high, saturation of the light output occurs due to gain saturation caused by spatial hole burning, whereby the high output operation is impeded. In addition, in the wide-stripe laser, although no kink occurs since the transverse mode of the waveguide light is a multimode, the shape of the transverse mode oscillation greatly changes to be unstable due to the spatial gain saturation caused by spatial hole burning. Further, in a light source for a laser display, although the light condensing characteristic up to the diffraction limit is not required as described above, when the transverse mode changes at a low frequency, the brightness of the display changes temporally, resulting in a problem that the brightness, color, and contrast cannot be accurately reproduced.
A measure to prevent the transverse mode from becoming unstable due to spatial hole burning is to narrow the stripe width. That is, as the stripe width becomes narrower, the expansion of the distribution of carriers which are injected into the active layer and the expansion in the transverse direction of the intensity distribution of the light that is induced in the active layer are relatively narrowed, and thereby, occurrence of unstable transverse mode that is caused by the spatial hole burning is prevented.
However, if the stripe width is uniformly narrowed over the entirety of the resonator, the series resistance of the element increases, and the driving voltage of the element undesirably increases. Especially, since reliability of a nitride-base semiconductor laser largely depends on the driving voltage and driving current, increase in the driving voltage must be restricted as much as possible. Further, since the light density in the waveguide increases when the stripe width is narrowed, it becomes difficult to realize high output characteristic of 100 mW or more.
In order to solve these problems, Patent Document 1 (Japanese Published Patent Application No. 2000-357842 (Pages 5-7, FIG. 1)) discloses a semiconductor laser having a taper region wherein the width of the stripe decreases from the center of the resonator toward the both end facets of the resonator. This semiconductor laser can provide laser oscillation with a stable transverse mode without excessively increasing the driving voltage for the element, as compared with the conventional laser structure in which the stripe width is uniformly narrowed.
Meanwhile, Patent Document 2 (Japanese Patent Publication No. 1862544 (Pages 2-3, FIG. 2)) discloses a semiconductor laser in which an electrode is divided into plural electrodes along an axis direction of a resonator, and voltages applied to the respective electrodes are controlled so that an injection current distribution according to a light intensity distribution of a higher light intensity is obtained in the center of the laser than in the other part of the laser, thereby realizing a semiconductor laser that achieves a stable transverse mode operation and an increased fabrication yield.
Further, Non-Patent Document 1 (“Semiconductor Laser” written and edited by Kenichi Iga, 1st edition, Ohmsha Ltd., Oct. 25, 1994, p. 238) discloses that a method of making the reflectivities at resonator facets asymmetrical is effective for high power output of a semiconductor laser. This method is common in a semiconductor laser used for writing of an optical disc, and the facets constituting the resonator is coated with dielectric films to make the reflectivity at the facets asymmetrical. To be specific, between the facets constituting the resonator, the reflectivity at the front facet from which laser light is emitted is lowered while the reflectivity at the rear facet opposite to the front facet is increased. For example, the reflectivity at the front facet is 10%, and the reflectivity at the rear facet is 90%. The reflectivity of the dielectric multilayer can be controlled according to the refractive index of the dielectric material, the thickness thereof, and the number of layers to be laminated.